Interpretive Summary: Powdery scab causes a disease in potato that is not controllable with fungicides. It affects the potato plant mostly by damaging the root system, but also by blemishing tubers. The yield reduction due to root impairment is often the most important affect. This study shows that there are breeding parents and varieites of potato that evidence less root damage as judged by the amount of galling that occurs on the tubers. Galling is excessive growth of roots caused by the fungus. Galls contain cystosori, or the resting stage of the fungus. It is the primary way that fungus is left in the soil, dormant for many years until a potato crop is again planted. Then the cysotosori provide a reservoir of new inoculum. This studies showed that there is considerable instability in the expression of resistance to galling. However, potato genotypes of potato could be identified that were consistently resistant in different environments. The resistance of these genotypes would prevent yield losses, sometimes as high as 25 per cent, if it was incorporated into new varieties. Powdery scab is one of the most important emerging diseases in the world and in the Columbia Basin, in particular.

Technical Abstract:
Spongospora subteranea, the causal agent of potato powdery scab is becoming increasingly important worldwide. Little is known about the genetic basis of resistance to this disease. The present study tested the hypothesis that potato genotypes with stable genetic resistance to "Spongospora root galling" were present in potato germplasm. Root galling index values of 24 genotypes screened for resistance in four field trials (environments) in 2004 and 2005 in Washington State and Idaho were analyzed. Genotypes tested included five resistant, four industry standards and advanced selections from the USDA-ARS, Prosser, WA program. Broad-sense heritability was calculated as 0.76 with a 95% confidence interval of 0.55 to 0.89, indicating a fairly high genetic component of the trait. Of the 24 genotypes that were tested, eight showed no genotype*environment interactions while six of the remainder had significant variance (i.e., they were unstable) after removal of genotype*environment variance. Among the five resistant genotypes, PA95B2-4 was stable, and PA98N5-2, PA98NM38-1, PO94A009-7 and POR00HG5-1 were stable after the removal of environmental heterogeneity. Among the four industry standards, Shepody was unstable, whereas Ranger Russet, Russet Burbank and Umatilla Russet were stable after the removal of genotype*environment variance. Stable resistance to "Spongospora root galling" was identified. A large portion of the variation was genetic, which will enable breeders to use resistant and stable potato genotypes as parents in future breeding to develop superior commercial potato cultivars with resistance to "Spongospora root galling".